Search

Background: The blood-brain barrier (BBB) contains tight junctions (TJs) which reduce the space between adjacent endothelial cells lining the fine capillaries of the microvasculature of the brain to form a selective and regulatable barrier., Methods: Using a hydrodynamic approach, we delivered siRNA targeting the TJ protein claudin-5 to the endothelial cells of the BBB in mice., Results: We have shown a significant decrease in claudin-5 mRNA levels 24 and 48 hours post-delivery of siRNA, with levels of protein expression decreasing up to 48 hours post-injection compared to uninjected, phosphate-buffered saline (PBS)-injected and non-targeting siRNA-injected mice. We observed increased permeability at the BBB to molecules up to 742 Da, but not 4400 Da, using tracer molecule perfusion and MRI analysis. To illustrate the functional efficacy of size-selective and transient barrier opening, we have shown that enhanced delivery of the small neuropeptide thyrotropin-releasing hormone (TRH) (MW 360 Da) to the brains of mice 48 hours post-injection of siRNA targeting claudin-5 significantly modifies behavioural output., Conclusions: These data demonstrate that it is now possible to transiently and size-selectively open the BBB in mice, allowing in principle the delivery of a wide range of agents for the establishment and treatment of experimental mouse models of neurodegenerative, neuropsychiatric and malignant diseases., ((c) 2008 John Wiley & Sons, Ltd.)

The cause of Parkinson's disease (PD) is unknown. Genetic susceptibility and exposure to environmental toxins contribute to specific neuronal loss in PD. Decreased blood-brain barrier (BBB) P-glycoprotein (P-gp) efflux function has been proposed as a possible causative link between toxin exposure and PD neurodegeneration. In the present study BBB P-gp function was investigated in vivo in 10 early stage PD patients and 8 healthy control subjects using (R)-[(11)C]-verapamil and PET. Cerebral volume of distribution (V(d)) of verapamil was used as measure of P-gp function. Both region of interest (ROI) analysis and voxel analysis using statistical parametric mapping (SPM) were performed to assess regional brain P-gp function. In addition, MDR1 genetic polymorphism was assessed. In the present study, a larger variation in V(d) of (R)-[(11)C]-verapamil was seen in the PD group as compared to the control group. However, decreased BBB P-gp function in early stage PD patients could not be confirmed.

Previously, it was shown that low-intensity focused ultrasound pulses applied along with an ultrasound contrast agent results in temporary blood-brain barrier (BBB) disruption. This effect could be used for targeted drug delivery in the central nervous system. This study examined the effects of burst length, pulse repetition frequency (PRF), and ultrasound contrast agent dose on the resulting BBB disruption. One hundred nonoverlapping brain locations were sonicated through a craniotomy in experiments in 26 rabbits (ultrasound frequency: 0.69 MHz, burst: 0.1, 1, 10 ms, PRF: 0.5, 1, 2, 5 Hz, duration: 20 s, peak negative pressure amplitude: 0.1 to 1.5 MPa, Optison dosage 50, 100, 250 microl/kg). For each sonication, BBB disruption was evaluated using contrast-enhanced magnetic resonance imaging. The BBB disruption threshold (the pressure amplitude yielding a 50% probability for BBB disruption) was determined using probit regression for the three burst lengths tested. Tissue effects were examined in light microscopy for representative locations with similar amounts of contrast enhancement from each group. While changing the PRF or the Optison dosage did not result in a significant difference in the magnitude of the BBB disruption (p > 0.05), reducing the burst length resulted in significantly less contrast enhancement (p < 0.01). The BBB disruption thresholds were estimated to be 0.69, 0.47 and 0.36 MPa for 0.1, 1 and 10 ms bursts, respectively. No difference was detected in histology between any experimental groups. This data suggests that over the range of parameters tested, BBB disruption is not affected by PRF or ultrasound contrast agent dose. However, both the BBB disruption magnitude and its threshold depend on the burst length.

Introduction: A novel [18F]-radiolabelled phenoxyanilide, [18F]-FEPPA, has been synthesized and evaluated, in vitro and ex vivo, as a potential positron emission tomography imaging agent for the peripheral benzodiazepine receptor (PBR)., Methods: [18F]-FEPPA and two other radiotracers for imaging PBR, namely [11C]-PBR28 and [11C]-PBR28-d3, were synthesised and evaluated in vitro and ex vivo as potential PBR imaging agents., Results: [18F]-FEPPA is efficiently prepared in one step from its tosylate precursor and [18F]-fluoride in high radiochemical yields and at high specific activity. FEPPA displayed a Ki of 0.07 nM for PBR in rat mitochondrial membrane preparations and a suitable lipophilicity for brain penetration (log P of 2.99 at pH 7.4). Upon intravenous injection into rats, [18F]-FEPPA showed moderate brain uptake [standard uptake value (SUV) of 0.6 at 5 min] and a slow washout (SUV of 0.35 after 60 min). Highest uptake of radioactivity was seen in the hypothalamus and olfactory bulb, regions previously reported to be enriched in PBR in rat brain. Analysis of plasma and brain extracts demonstrated that [18F]-FEPPA was rapidly metabolized, but no lipophilic metabolites were observed in either preparation and only 5% radioactive metabolites were present in brain tissue extracts. Blocking studies to determine the extent of specific binding of [18F]-FEPPA in rat brain were problematic due to large perturbations in circulating radiotracer and the lack of a reference region., Conclusions: Further evaluation of the potential of [18F]-FEPPA will require the employment of rigorous kinetic models and/or appropriate animal models.

Imidazopyridineacetoamide 5-8, a series of novel and potentially selective peripheral benzodiazepine receptor (PBR) ligands with affinities comparable to those of known PBR ligands, was investigated. Radiosyntheses of [11C]5, 6, 7 or 8 was accomplished by N-methylation of the corresponding desmethyl precursors with [11C]methyl iodide in the presence of NaH in dimethylformamide (DMF), resulting in 25% to 77% radiochemical yield and specific activitiy of 20 to 150 MBq/nmol. Each of the labeled compounds was injected in ddY mice, and the radioactivity and weight of dissected peripheral organs and brain regions were measured. Organ distribution of [11C]7 was consistent with the known PBR distribution. Moreover, [11C]7 showed the best combination of brain uptake and PBR binding, leading to its high retention in the olfactory bulb and cerebellum, areas where PBR density is high in mouse brain. Coinjection of PK11195 or unlabeled 7 significantly reduced the brain uptake of [11C]7. These results suggest that [11C]7 could be a useful radioligand for positron emission tomography imaging of PBRs.

Aim: To determine the incidence and characteristics of maculopathy associated with blood-brain barrier disruption (BBBD) which is used in treating primary central nervous system lymphoma (PCNSL)., Methods: Files of all 56 patients with PCNSL, with or without intraocular lymphoma (IOL), treated at Hadassah University Hospital during the years 1997-2007 were reviewed. Data on 46 patients for whom we had documentation of ocular examination were studied. Those who were alive at the time of the data collection were invited for further evaluation of the presence or absence of maculopathy. The patients were divided into four groups according to treatment protocol. Group 1: systemic intravenous chemotherapy; Group 2: systemic intravenous chemotherapy and intravitreal methotrexate (MTX); Group 3: systemic intra-arterial (IA) chemotherapy and BBBD; Group 4: systemic IA chemotherapy, BBBD, and intravitreal MTX., Results: Of the 23 patients of Groups 1 and 2 who were not treated by BBBD, none developed maculopathy. Of those 23 patients who were treated by BBBD, 12 of 17 (70.5%) in Group 3 and three of six (50%) in Group 4, for a total of 15 of 23 (65.2%) developed maculopathy. The maculopathy did not significantly affect visual acuity in any of them., Conclusions: BBBD may cause maculopathy in almost two-thirds of patients treated for PCNSL, without affecting significantly the visual acuity. Intravitreal injection of MTX, according to the protocol which we apply, is not associated with maculopathy.

The blood-brain barrier (BBB) is a complex functional barrier composed of endothelial cells, pericytes, astrocytic endfeets and neuronal cells. This highly organized complex express a selective permeability for molecules that bear, amongst other parameters, adequate molecular weight and sufficient liposolubility. Unfortunately, very few therapeutic agents currently available do cross the BBB and enters the CNS. As the BBB limitation is more and more acknowledged, many innovative surgical and pharmacological strategies have been developed to circumvent it. This review focuses particularly on the osmotic opening of the BBB, a well-documented approach intended to breach the BBB. Since its inception by Rapoport in 1972, pre-clinical studies have provided important information on the extent of BBB permeation. Thanks to Neuwelt and colleagues, the osmotic opening of the BBB made its way to the clinic. However, many questions remain as to the detailed physiology of the procedure, and its best application to the clinic. Using different tools, amongst which MRI as a real-time in vivo characterization of the BBB permeability and CNS delivery, we attempt to better define the osmotic BBB permeabilization physiology. These ongoing studies are described, and data related to spatial and temporal distribution of a molecule after osmotic BBB breaching, as well as the window of BBB permeabilization, are discussed. We also summarize recent clinical series highlighting promising results in the application of this procedure to maximize delivery of chemotherapy in the treatment of brain tumor patients.

We have developed a method to use low-intensity focused ultrasound pulses combined with an ultrasound contrast agent to produce temporary blood-brain barrier disruption (BBBD). This method could provide a means for the targeted delivery of drugs or imaging agents into the brain. In all our previous work, we used Optison as the ultrasound contrast agent. The purpose of this study was to test the feasibility of using the contrast agent Definity for BBBD. A total of 36 non-overlapping locations were sonicated through a craniotomy in experiments in the brains of nine rabbits (four locations per rabbit; ultrasound [US] frequency: 0.69 MHz; burst: 10 ms; pulse repetition frequency (PRF): 1 Hz; duration: 20 s). The peak negative pressure amplitude ranged from 0.2 to 1.5 MPa. An additional 11 locations were sonicated using Optison at pressure amplitude of 0.5 MPa. Definity and Optison dosages were the same as those used clinically for ultrasound imaging: 10 and 50 microl/kg, respectively. The probability for BBBD (determined using MRI contrast agent enhancement) as a function of pressure amplitude was similar to that found earlier with Optison. For both agents, the probability was estimated to be 50% at 0.4 MPa using probit regression. Histologic examination revealed small, isolated areas of extravasated erythrocytes in some locations. At 0.8 MPa and higher, these areas were sometimes accompanied by tiny (dimensions of 100 microm or less) regions of damaged brain parenchyma. The magnitude of the BBBD was larger with Optison than with Definity at 0.5 MPa (signal enhancement: 13.3% +/- 4.4% vs. 8.4% +/- 4.9%; p = 0.04). In addition, more areas with extravasated erythrocytes were observed with Optison (5.0 +/- 3.5 vs. 1.4 +/- 1.9 areas with extravasation in histology section with largest effect; p = 0.03). We concluded that BBBD is possible using Definity at the dosage of contrast agent and the acoustic parameters tested in this study. The probability for BBBD as a function of pressure amplitude and the type of acute tissue effects were similar to what has been observed using Optison. However, under the experimental conditions used in this study, Optison produced a larger effect for the same acoustic pressure amplitude.

Objective: The aim of this study was to quantify the effect of hypertonic saline solution on contused and noncontused brain tissue in patients with traumatic brain injury. We hypothesize that hypertonic saline would increase the volume of brain contusion while decreasing the volume of noncontused hemispheric areas., Design: Prospective observational study., Setting: Neurosciences critical care unit of a university hospital., Patients: Fourteen traumatic brain injury patients with increased intracranial pressure., Interventions: A computed tomography scan was performed before and after a 20-min infusion of 40 mL of 20% saline., Measurements and Main Results: The volume, weight, and specific gravity of contused and noncontused hemispheric areas were assessed from computed tomography DICOM images by using a custom-designed software (BrainView). Physiologic variables and natremia were measured before and after infusion. Hypertonic saline significantly increased natremia from 143 +/- 5 to 146 +/- 5 mmol/L and decreased intracranial pressure from 23 +/- 3 to 17 +/- 5 mm Hg. The volume of the noncontused hemispheric areas decreased by 13 +/- 8 mL whereas the specific gravity increased by 0.029 +/- 0.027%. The volume of contused hemispheric tissue increased by 5 +/- 5 mL without any con-comitant change in density. There was a wide interindividual variability in the response of the noncontused hemispheric tissue with changes in specific gravity varying between -0.0124% and 0.0998%., Conclusions: Three days after traumatic brain injury, the blood- brain barrier remains semipermeable in noncontused areas but not in contusions. Further studies are needed to tailor the use of hypertonic saline in patients with traumatic brain injury according to the volume of contusions assessed on computed tomography.

Background: Malignant middle cerebral artery (MMCA) infarction is associated with severe brain edema which may lead to a rapid deterioration of consciousness, increase of intracranial pressure, brain midline shift and finally, herniation. We examined the correlation between the degree of the blood-brain barrier (BBB) permeability and MMCA., Methods: Twenty-five consecutive patients (17 men and 8 women, mean age 62.1+/-10.1) were included in the study. Each patient had a daily clinical examination, and the neurological deficits were scored using NIHSS score. A CT without contrast material was performed in all patients. (99m)Tc-DTPA SPECT was performed at 36 h after the stroke. A quantitative index of BBB breakdown (disruption index) was calculated., Results: The mean volume of stroke was 138+/-87 cm(3). The mean DTPA disruption index was 6.6+/-4.6 (range 1.0-21.0). The mean NIHSS score was 14+/-4 (p=0.2). Five of 25 patients had brain herniation as evidenced on brain CT. The volume of stroke was only marginally elevated in patients with herniation (p=0.062). All patients showed significant, inverse correlation between NIHSS score and DTPA uptake (r=-0.43, p=0.033). There was a significant correlation between the extent of DTPA distribution (more than one vascular territory) and the occurrence of herniation (p<0.001)., Conclusions: DTPA-SPECT imaging is a reliable complementary predictive tool in patients with an MCA stroke. The specific pattern found on DTPA SPECT, compatible with diffuse BBB disruption, may be of value in predicting "malignant MCA."

Unlabelled: P-glycoprotein (P-gp) is a membrane protein that functions as an adenosine triphosphate-dependent efflux pump for xenobiotics at the blood-brain barrier (BBB). Polymorphisms of MDR1 gene have been reported to be associated with the expression level of P-gp. (11)C-Verapamil is considered to be one of the suitable radioligands for evaluating P-gp functions. However, the metabolites of verapamil might complicate the quantitative analysis because of their possible brain penetration. In the present study, we investigated the P-gp functional differences at the BBB between the haplotypes (1236TT, 2677TT, 3435TT vs. 1236CC, 2677GG, 3435CC) of the MDR1 gene with different quantitative analyses of (11)C-verapamil., Methods: Thirty-three healthy male volunteers were enrolled in this study after identification of the haplotypes of the MDR1 gene. Brain PET scans with (11)C-verapamil were performed for 16 min. Integration plot analysis, which yields brain uptake clearance, was performed with the first 3-min data. Integration plot analysis was then compared with several other quantitative analyses with 16-min data (1-input, 1-tissue compartment model, and the area under the curve ratio (AUC(ratio)) between brain and plasma)., Results: In the integration plot, there was no difference in the absolute values of brain uptake clearance (CL(uptake)) between the haplotypes; CL(uptake) of (11)C-verapamil for the haplotypes (1236TT, 2677TT, 3435TT vs. 1236CC, 2677GG, 3435CC) were 0.053 +/- 0.011 and 0.051 +/- 0.011 mL/g/min, respectively. CL(uptake) of (11)C-verapamil in the integration plot was significantly correlated with K1 and DV(K1/k2) (DV is distribution volume; K1 and k2 are plasma and tissue rate constants) in the 1-input, 1-tissue compartment model and the AUC(ratio)., Conclusion: On the basis of the several quantitative analyses of (11)C-verapamil, the assumption that the function of P-gp at the BBB is different between the haplotypes (3 single nucleotide polymorphisms: C1236T, G2677T, and C3435T) of MDR1 gene was not supported.

Objective: Endothelial tight junctions form the main barrier of the blood-brain barrier (BBB). In human hemangioblastomas, cyst formation is a common and important clinical manifestation. Although most researchers consider that the cyst formation in hemangioblastomas may be caused by the breakdown of the BBB, the underlying molecular mechanisms for cyst formation remain unknown. At present, there are few reports about the change of tight junctions in microvessel endothelium of human hemangioblastomas. The purpose of this research is to investigate the change of tight junction and its major molecular components in microvessel endothelium of human hemangioblastomas., Methods: Twenty-four consecutive patients with cerebellar hemangioblastomas were studied. Tight junctions in the microvessels of hemangioblastomas and the control brain were examined by electron microscopy. Immunohistochemistry and double immunofluorescent microscopy were used to analyze the expression of CLN5 and its relationship with astrocytic endfeet in the control brain and hemangioblastomas. Quantitative real-time reverse-transcriptase polymerase chain reaction and Western blots were used to investigate the expression level of CLN5 in hemangioblastomas. Triple immunofluorescent microscopy was used to analyze the coexpression of vascular endothelial growth factor, vascular endothelial growth factor-R1, and placenta growth factor on microvessels of hemangioblastomas. Clinical and experimental data were correlated and analyzed by the one-way analysis of variance, Kruskal-Wallis test, and Spearman rank correlation test., Results: In the control brain, the paracellular cleft between adjacent endothelial cells is sealed by continuous strands of tight junctions. In cystic hemangioblastomas, a significant paracellular cleft could be found between adjacent endothelial cells. Some endothelial cells were connected with adherens junction and no tight junction was found between them. Compared with the control brain, expression of CLN5 was decreased in cystic hemangioblastomas (P < 0.05). Phosphorylated CLN5 was detected in most hemangioblastomas, but not in the control brain. Microvessels in hemangioblastomas showed a significant absence of astrocytic endfeet. Coexpression of vascular endothelial growth factor, vascular endothelial growth factor-R1, and placenta growth factor was detected in the endothelial cells. The Spearman rank correlation test showed a significant correlation between a greater degree of CLN5 expression and less morphological cystic formation in these patients studied (correlation coefficient = -0.520; P = 0.009)., Conclusion: The continuity of tight junctions of the BBB is interrupted in human cerebellar hemangioblastomas. Significant absence of astrocytic endfeet and tight junctions can be found in microvessels of hemangioblastomas, which may lead to the breakdown of the BBB in these tumors. These findings suggest that the absence of tight junctions might play a role in cyst formation of hemangioblastomas.

Unlabelled: P-glycoprotein in the blood-brain barrier (BBB) has been found to be associated with several types of neurologic damage. (11)C-Verapamil has been used for in vivo imaging of P-glycoprotein function in the BBB by PET, but metabolites in plasma complicate the quantitative analysis of human studies. In this study, we validated the quantification methods of (11)C-verapamil transfer from plasma to the brain in humans., Methods: The transfer rate constant from plasma to the brain, K(1), was estimated by nonlinear least squares (NLS) with a 2-input compartment model, including the permeation of the main metabolite in plasma at the BBB, and with a 1-input compartment model using only 15-min data that contained little metabolite in plasma. K(1) was also estimated by graphical analysis of an integration plot that uses only early-time data, before the appearance of metabolites, and the estimated K(1) was compared with that obtained by the NLS method. In the simulation study, the reliability of parameter estimates in the graphical analysis method was investigated for various values of rate constants, time ranges of parameter estimations, and noise levels., Results: (11)C-Verapamil in plasma gradually converted to its metabolites, and about 45% of the radioactivity in the plasma specimen was associated with (11)C-verapamil metabolites at 30 min after injection. Although K(1) estimated from graphical analysis was slightly smaller than that by NLS, there was strong correlation among the K(1) values obtained by these 3 methods. In the simulation study, for graphical analysis, the differences between the true and mean of K(1) estimates became larger and the coefficient of variation (COV) of K(1) estimates became smaller as the end time of linear regression became later. The COV of graphical analysis was almost equal to that of NLS with the 1-input compartment model., Conclusion: The transfer of (11)C-verapamil from plasma to the brain was able to be quantitatively estimated by graphical analysis because this method can provide K(1) from the data of the initial few minutes without considering the effect of the metabolites in plasma.

Studies on the development of imaging agents for targeting neuroreceptors is an area of considerable interest owing to the limited availability of specific as well as selective radiolabeled agents. Therefore, with an aim of developing a receptor-specific agent, iminodiacetic acid (IDA) derivative of 5-hydroxy tryptamine viz., HTIDA has been synthesized. HTIDA could be radiolabeled with the synthon [(99m)Tc(CO)(3)(H(2)O)(3)](+) in >98% yield. The biodistribution studies in normal Swiss mice showed that the (99m)Tc(CO)(3)-HTIDA crosses the blood-brain barrier successfully with a brain uptake of 0.5%ID/g at 5min post injection. The other relevant observations from biodistribution studies included no significant uptake in any other organ and fast clearance from blood, lungs and liver.

Antibody-based anticancer agents are promising chemotherapeutic agents. Among these agents, Herceptin (trastuzumab), a humanized anti-human epidermal growth factor receptor 2 (HER2/c-erbB2) monoclonal antibody, has been used successfully in patients with breast cancer. However, in patients with brain metastasis, the blood-brain barrier limits its use, and a different delivery method is needed to treat these patients. Here, we report that Herceptin can be delivered locally and noninvasively into the mouse central nervous system through the blood-brain barrier under image guidance by using an MRI-guided focused ultrasound blood-brain barrier disruption technique. The amount of Herceptin delivered to the target tissue was correlated with the extent of the MRI-monitored barrier opening, making it possible to estimate indirectly the amount of Herceptin delivered. Histological changes attributable to this procedure were minimal. This method may represent a powerful technique for the delivery of macromolecular agents such as antibodies to treat patients with diseases of the central nervous system.

Introduction: P-glycoprotein (P-gp) is an efflux transporter responsible for the transport of various drugs across the blood-brain barrier (BBB). Loss of P-gp function with age may be one factor in the development and progression of neurodegenerative diseases. The aim of this study was to assess the effect of aging on BBB P-gp function. Furthermore, the relationship between BBB P-gp activity and peripheral P-gp activity in CD3-positive leukocytes was investigated. Finally, plasma pharmacokinetics of carbon 11-labeled (R)-verapamil was evaluated., Methods: (R)-[(11)C]verapamil and positron emission tomography were used to assess gray matter P-gp function. Because (R)-[(11)C]verapamil is a substrate for P-gp, the volume of distribution of (R)-[(11)C]verapamil in the brain inversely reflects P-gp function in the BBB., Results: Mean volume of distribution values for 5 young healthy volunteers (age range, 21-27 years) and 5 elderly healthy volunteers (age range, 59-68 years) were 0.62+/-0.10 and 0.73+/-0.07, respectively (P=.03). The activity index of P-gp activity in CD3-positive leukocytes was 2.88+/-0.77 in young volunteers and 1.76+/-0.58 in elderly volunteers (P=.02)., Conclusion: This study showed decreased P-gp activity during aging. Consequently, the brain may be exposed to higher drug and toxin levels in elderly subjects.

Introduction: The positron emission tomography (PET) tracers (18)F-fluoro-ethyl-L: -tyrosine (FET), (18)F-fluorocholine (N,N-dimethyl-N-[(18)F]fluoromethyl-2-hydroxyethylammonium (FCH]) and (18)F-fluoro-2-deoxyglucose (FDG) are used in the diagnosis of brain tumours. The aim of this study was threefold: (a) to assess the uptake of the different tracers in the F98 rat glioma, (b) to evaluate the impact of blood-brain barrier (BBB) disruption and microvessel density (MVD) on tracer uptake and (c) to compare the uptake in the tumours to that in the radiation injuries (induced by proton irradiation of healthy rats) of our previous study., Methods: F98 gliomas were induced in 26 rats. The uptake of FET, FCH and FDG was measured using autoradiography and correlated with histology, disruption of the BBB and MVD., Results: The mean FET, FCH and FDG standardised uptake values (SUVs) in the tumour and the contralateral normal cortex (in parentheses) were 4.19+/-0.86 (1.32+/-0.26), 2.98+/-0.58 (0.51+/-0.11) and 11.02+/-3.84 (4.76+/-1.77) respectively. MVD was significantly correlated only with FCH uptake. There was a trend towards a negative correlation between the degree of BBB disruption and FCH uptake and a trend towards a positive correlation with FET uptake. The ratio of the uptake in tumours to that in the radiation injuries was 1.97 (FCH), 2.71 (FET) and 2.37 (FDG)., Conclusion: MVD displayed a significant effect only on FCH uptake. The degree of BBB disruption seems to affect the accumulation of FET and FCH, but not FDG. Mean tumour uptake for all tracers was significantly higher than the accumulation in radiation injuries.

In animal studies, acute antipsychotic treatment was shown to enhance striatal DOPA-decarboxylase (DDC) activity. However, this phenomenon has not been demonstrated in humans by positron emission tomography (PET). Therefore, we investigated acute haloperidol effects on DDC activity in humans using [18F]fluorodopa (FDOPA) PET. Nine healthy volunteers were scanned with FDOPA in drug-free baseline conditions and after 3 days of haloperidol treatment (5 mg/day). A continuous performance test (CPT) was administered in both conditions. The net blood-brain clearance of FDOPA (K(in)app) in striatum, mesencephalon, and medial prefrontal cortex was calculated by volume-of-interest analysis. The macroparameter K(in)app is a composite of several kinetic terms defining the distribution volume of FDOPA in brain (V(e)D) and the relative activity of DOPA decarboxylase (k3D). Therefore, compartmental kinetic analysis was used to identify the physiological basis of the observed changes in K(in)app period. The magnitude of K(in)app was significantly increased in the putamen (18%) and mesencephalon (36%). Furthermore, V(e)D in the brain was increased by 15%. Increments of k3(D) in the basal ganglia did not attain statistical significance. The significant worsening of CPT results did not correlate with changes in FDOPA utilization. The present PET results indicate potentiation of FDOPA utilization in human basal ganglia by acute haloperidol treatment, apparently due to increased availability throughout the brain. The stimulation of DDC cannot be excluded due to insufficient statistical power in the estimation of k3(D) changes.

Positron emission tomography (PET) with the drug radiolabelled allows a direct measurement of brain or other organ kinetics, information which can be essential in drug development. Usually, however, a PET-tracer is administered intravenously (i.v.), whereas the therapeutic drug is mostly given orally or by a different route to the PET-tracer. In such cases, a recalculation is needed to make the PET data representative for the alternative administration route. To investigate the blood-brain barrier penetration of a drug (zolmitriptan) using dynamic PET and by PK modelling quantify the brain concentration of the drug after the nasal administration of a therapeutic dose. [11C]Zolmitriptan at tracer dose was administered as a short i.v. infusion and the brain tissue and venous blood kinetics of [11C]zolmitriptan was measured by PET in 7 healthy volunteers. One PET study was performed before and one 30 min after the administration of 5 mg zolmitriptan as nasal spray. At each of the instances, the brain radioactivity concentration after subtraction of the vascular component was determined up to 90 min after administration and compared to venous plasma radioactivity concentration after correction for radiolabelled metabolites. Convolution methods were used to describe the relationship between arterial and venous tracer concentrations, respectively between brain and arterial tracer concentration. Finally, the impulse response functions derived from the PET studies were applied on plasma PK data to estimate the brain zolmitriptan concentration after a nasal administration of a therapeutic dose. The studies shows that the PET data on brain kinetics could well be described as the convolution of venous tracer kinetics with an impulse response including terms for arterial-to-venous plasma and arterial-to-brain impulse responses. Application of the PET derived impulse responses on the plasma PK from nasal administration demonstrated that brain PK of zolmitriptan increased with time, achieving about 0.5 mg/ml at 30 min and close to a maximum of 1.5 mg/ml after 2 hr. A significant brain concentration was observed already after 5 min. The data support the notation of a rapid brain availability of zolmitriptan after nasal administration.

Significant new developments in neurovascular ultrasound include molecular approaches to diagnostics and therapy. Addition of targeted ligands to microbubbles, has opened new avenues for the identification of vascular injury. This is because the molecular signatures of overexpressed adhesion molecules such as the integrin alphavbeta3, ICAM-1, and fibrinogen receptor GPIIb/II can be used to localize contrast agents through the use of complementary receptor ligands. Recent experiments have demonstrated the feasibility of microbubble-ultrasound-enhanced gene therapy to the brain. This new technology holds the promise of delivering genes more selectively than other methods and less invasively than direct injection. Microbubbles may also be employed as carriers of gene agents. The ability to focus ultrasound and cause local cavitation with these carriers may provide a new tool for gene therapy. Fortuitously, the intact blood-brain barrier (BBB), a major limitation in using genes for therapy of brain disease, can be opened with ultrasound. This localized, transient, and reversible opening of the BBB with ultrasound can provide an anatomically selective and targeted gene delivery. Future developments in neurovascular ultrasound will include improvements in technologies for ligand attachment to microbubbles, better methods for imaging targeted ultrasound agents in the brain, and optimization of ultrasound-mediated gene delivery.

The selective dopamine D(3) receptor ligands N-4-[4-[(2,3-dichlorophenyl)piperazin-1-yl]butyl]1-methoxy-2-naphthalencarboxamide (1) and N-4-[4-[(2,3-dichlorophenyl)piperazin-1-yl]butyl]-7-methoxy-2-benzofurancarboxamide (2) were labeled with (11)C (t(1/2) = 20.4 min) as potential radioligands for the noninvasive assessment of the dopamine D(3) neurotransmission system in vivo with positron emission tomography (PET). The radiosynthesis consisted in an O-methylation of the des-methyl precursors N-[4-[4-(2,3-dichlorophenyl)piperazin-1-yl]butyl]-1-hydroxy-2-naphthalenecarboxamide (3) and N-[4-[4-(2,3-dichlorophenyl)piperazin-1-yl]butyl]-7-hydroxy-2-benzofurancarboxamide (4) with [(11)C]methyl iodide using tBuOK/HMPA and KOH/DMSO, respectively. The radiotracers [(11)C]1 and [(11)C]2 were obtained in 35 min with over 99% radiochemical purity, 74 +/- 37 GBq/mumol of specific radioactivity, 13% and 26% radiochemical yield (EOB, decay-corrected). Distribution studies in rats demonstrated that the new tracers [(11)C]1 and [(11)C]2 cross the blood-brain barrier and localize in the brain. However, the kinetics of cerebral uptake did not reflect the regional expression of the D(3) receptors. Despite their in vitro pharmacological profile, [(11)C]1 and [(11)C]2 do not display an in vivo behavior suitable to image D(3) receptor expression using PET.

Although it has long been presumed that d-amino acids are uncommon in mammalians, substantial amounts of free d-serine have been detected in the mammalian brain. d-Serine has been demonstrated to be an important modulator of glutamatergic neurotransmission and acts as an agonist at the strychnine-insensitive glycine site of N-methyl-d-aspartate receptors. The blood-to-brain transfer of d-serine is thought to be extremely low, and it is assumed that d-serine is generated by isomerization of l-serine in the brain. Stimulated by the observation of a preferred transport of the d-isomer of proline at the blood-brain barrier, we investigated the differential uptake of [3H]-d-serine and [3H]-l-serine in the rat brain 1 h after intravenous injection using quantitative autoradiography. Surprisingly, brain uptake of [3H]-d-serine was significantly higher than that of [3H]-l-serine, indicating a preferred transport of the d-enantiomer of serine at the blood-brain barrier. This finding indicates that exogenous d-serine may have a direct influence on glutamatergic neurotransmission and associated diseases.

Unlabelled: This study evaluated the pharmacokinetics and biodistribution of 4-borono-2-(18)F-fluoro-l-phenylalanine ((18)F-FBPA) after intracarotid injection and with blood-brain barrier disruption (BBB-D) in F98 glioma-bearing F344 rats. The pharmacokinetics of l-p-boronophenylalanine (BPA) and (18)F-FBPA following different administration routes were compared to demonstrate the optimal delivery route and the time period for thermal neutron irradiation., Methods: F98 glioma-bearing rats were injected intravenously or intracarotidly with (18)F-FBPA and BPA and with or without mannitol-induced hyperosmotic BBB-D. The boron concentration and (18)F radioactivity in tissues were determined by invasive (inductively coupled plasma mass spectroscopy, gamma-counting) and noninvasive PET methods., Results: The biodistributions of (18)F-FBPA and BPA in F98 glioma-bearing rats were similar after intracarotid administration with BBB-D. The accumulation of BPA and (18)F-FBPA in brain tumor and the tumor-to-ipsilateral brain ratios were the highest after intracarotid injection with BBB-D, whereas the retention of boron drugs in contralateral brains exhibited only nonsignificant differences compared with those after intracarotid injection without BBB-D and intravenous injection. The high boron concentration in brain tumor (76.6 mug/g) and the high tumor-to-ipsilateral brain ratio (6.3) may afford enough radiation doses to destroy the tumor cells while sparing the normal tissues in boron neutron capture therapy. The pharmacokinetic parameters of k(el), k(12), k(21), and V(1) for intracarotid injection of (18)F-FBPA with BBB-D derived from the open 2-compartment model are 0.0206 +/- 0.0018 min(-1), 0.0260 +/- 0.0016 min(-1), 0.0039 +/- 0.0003 min(-1), and 3.1 +/- 0.1 mL, respectively. The effect of BBB-D varied depending on the anesthetic agents used and the anesthetic conditions. A smaller degree of BBB-D and, thus, lower boron concentrations in tumor and ipsilateral brain were observed under isoflurane anesthesia than under ketamine anesthesia. The k(12)/k(21) ratio may serve as a good indication for evaluating the extent of BBB-D, tumor uptake, and tumor-to-brain ratio after intracarotid injection of boron compounds., Conclusion: Our findings provide important information for establishing an optimal treatment protocol when intracarotid injection with BPA after BBB-D is applied in clinical boron neutron capture therapy.

Focused ultrasound offers a method to disrupt the blood-brain barrier (BBB) noninvasively and reversibly at targeted locations. The purpose of this study was to test the safety of this method by searching for ischemia and apoptosis in areas with BBB disruption induced by pulsed ultrasound in the presence of preformed gas bubbles and by looking for delayed effects up to one month after sonication. Pulsed ultrasound exposures (sonications) were performed in the brains of 24 rabbits under monitoring by magnetic resonance imaging (MRI) (ultrasound: frequency = 1.63 MHz, burst length = 100 ms, PRF = 1 Hz, duration = 20 s, pressure amplitude 0.7 to 1.0 MPa). Before sonication, an ultrasound contrast agent (Optison, GE Healthcare, Milwaukee, WI, USA) was injected IV. BBB disruption was confirmed with contrast-enhanced MR images. Whole brain histologic examination was performed using haematoxylin and eosin staining for general histology, vanadium acid fuchsin-toluidine blue staining for ischemic neurons and TUNEL staining for apoptosis. The main effects observed were tiny regions of extravasated red blood cells scattered around the sonicated locations, indicating affected capillaries. Despite these vasculature effects, only a few cells in some of the sonicated areas showed evidence for apoptosis or ischemia. No ischemic or apoptotic regions were detected that would indicate a compromised blood supply was induced by the sonications. No delayed effects were observed either by MRI or histology up to 4 wk after sonication. Ultrasound-induced BBB disruption is possible without inducing substantial vascular damage that would result in ischemic or apoptotic death to neurons. These findings indicate that this method is safe for targeted drug delivery, at least when compared with the currently available invasive methods.

Pathologic changes in brain tissue during and after stroke may lead to injury of the blood-brain barrier (BBB) and subsequent hemorrhagic transformation (HT). In a rat model of HT, the apparent diffusion coefficient of water, cerebral blood flow, relaxation times, T(1) and T(2), and magnetization transfer (MT) related parameters (T(1sat), K(for) and the MT ratio) were repetitively measured during 3 h of focal ischemia and 2 h of reperfusion (n = 8). Areas of BBB opening were identified by sequential assay of the transcapillary influx of Gd-diethylenetriaminepentaacetic acid (Gd-DTPA) by MRI and (14)C-alpha-aminoisobutyric acid (AIB) by quantitative autoradiography. Ischemia-injured regions of interest were identified from the MRI data and divided into those with and without BBB opening. Of the several MRI parameters measured, the T(1sat) in the caudate-putamen and preoptic area during ischemia and the first 2 h of reperfusion correlated best with the regional pattern of BBB opening observed thereafter. These data suggest that an ipsilateral/contralateral T(1sat) ratio > 1.6 demarcates leakage of small molecules such as Gd-DTPA and AIB across the BBB. As to clinical relevance, the quantitation of MT parameters in acute stroke may enable the early detection of areas of BBB opening and potential HT., (Copyright 2005 Wiley-Liss, Inc.)

The ability of gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) enhanced MRI to localize and quantitate blood-brain barrier (BBB) opening was evaluated against quantitative autoradiographic (QAR) imaging of (14)C-alpha-aminoisobutyric acid (AIB) distribution. The blood-to-brain transfer constant (K(i)) for Gd-DTPA was determined by MRI in rats after 3 h of focal cerebral ischemia plus 2.5 h of reperfusion (n = 9), and that of AIB was determined by QAR shortly thereafter. Tissue regions of interest (ROIs) for Gd-DTPA leakage were identified by ISODATA segmentation of pre- and post-Gd-DTPA Look-Locker (L-L) T(1) maps. Patlak plots were constructed using time course of blood and tissue T(1) changes induced by Gd for estimating K(i). Among the nine rats, 14 sizable regions of AIB uptake were found; 13 were also identified by ISODATA segmentation. Although the 13 MRI-ROIs spatially approximated those of AIB uptake, the segmentation sometimes missed small areas of lesser AIB uptake that did not extend through more than 60% of the 2.0-mm-thick slice. Mean K(i)'s of AIB were highly correlated with those of Gd-DTPA across the 13 regions; the group means (+/-SD) were similar for the two tracers (7.1 +/- 3.3 x 10(-3) and 6.8 +/- 3.5 x 10(-3) ml.g(-1) . min(-1), respectively). In most instances, Gd-DTPA MRI accurately localized areas of BBB opening., (Copyright 2005 Wiley-Liss, Inc.)

Objective: In abstinent alcoholic patients, a low availability of dopamine D2/3 receptors in the ventral striatum and adjacent putamen was associated with a high level of craving for alcohol. Alcohol craving may also depend on presynaptic dysfunction of striatal dopamine production, which may contribute to the risk of relapse. In this study, positron emission tomography (PET) was used to compare dopamine synthesis capacity in the striatum in alcoholic patients and healthy comparison subjects., Method: Positron emission tomography (PET) was used to map the net blood-brain clearance of the dopa decarboxylase substrate 6-[18F]fluoro-l-dopa, an index of dopamine synthesis capacity, in the striatum of 12 detoxified male alcoholic patients and 13 age-matched healthy men. The parametric maps were correlated with results of an earlier [18F]desmethoxyfallypride PET study of dopamine D2/3 receptor availability in the same 12 alcoholic patients and in 12 of the healthy volunteers. Alcohol craving was measured with the Alcohol Craving Questionnaire. Patients were followed for 6 months, and alcohol intake was recorded., Results: The magnitude of net blood-brain clearance in the striatum did not differ significantly between detoxified alcoholic patients and the comparison subjects. However, a voxel-wise correlation analysis of net blood-brain clearance in the alcoholic patients linked low levels of dopamine synthesis capacity in the bilateral putamen with high levels of alcohol craving. After normalization of net blood-brain clearance maps to the voxel-wise estimates of dopamine D2/3 receptor availability, there was still a negative correlation with alcohol craving. Alcohol craving at the time of scanning was associated with high level of alcohol intake in the 6-month follow-up period., Conclusions: Simultaneous assay by PET of pre- and postsynaptic markers of dopamine neurotransmission indicated that a striatal dopamine deficit correlated with alcohol craving, which was associated with a high relapse risk.

As inversion-recovery (IR) technique improves T1 contrast at high field strength, signal enhancement by T1-shortening contrast media may be affected. To clarify the different enhancement properties at 3.0 T, the authors compared T1-weighted sequences. Twelve contrast-enhancing lesions were investigated by spin-echo (SE), inversion recovery fast spin-echo (IR-FSE), two-dimensional gradient-echo (2D GE), and magnetization-prepared three-dimensional gradient-echo (3D GE) sequences and evaluated by comparing signal-intensity enhancements within the lesions. In addition, signal-to-noise-ratios (SNR) and contrast-to-noise-ratios (CNR) were measured. On average, signal enhancement of the lesions amounted to 60% for SE, 57% for IR-FSE, 32% for 2D GE, and 35% for 3D GE images. CNR of gray matter versus white matter was significantly higher for IR SE and GE imaging than for genuine SE and 2D GE acquisitions (Wilcoxon test), while 2D GE imaging alone had an excellent SNR. As IR-FSE images provide an excellent CNR for gray and white matter in the brain and contrast enhancement performs almost similarly well compared with SE imaging, this technique appears to be well suited for T1-weighted neuroimaging without and with contrast enhancement at 3.0 T. However, the inherent blurring of the IR-FSE can lead to poor performance for very small lesions.

Background: Numerous knockout mouse studies have revealed that P-glycoprotein (P-gp) significantly limits drug distribution across the mouse blood-brain barrier (BBB). To determine the importance of P-gp at the human BBB, we developed a state-of-the-art, noninvasive, quantitative imaging technique to measure P-gp activity by use of carbon 11-labeled verapamil as the P-gp substrate and cyclosporine (INN, ciclosporin) as the P-gp inhibitor., Methods: In brief, 11C-verapamil (approximately 0.2 mCi/kg) was administered to healthy volunteers (n = 12 [6 women and 6 men]) as an intravenous infusion over a period of approximately 1 minute before and after at least a 1-hour infusion of cyclosporine (2.5 mg x kg(-1) x h(-1)). Arterial blood samples and brain positron emission tomography images were obtained at frequent intervals for 45 minutes. Both blood and plasma radioactivity contents were determined in each verapamil sample. The content of verapamil and its metabolites in the 20- and 45-minute plasma samples was determined by a rapid solid-phase extraction method. The brain uptake of 11C-radioactivity (brain area under the curve [AUCbrain ]/blood area under the curve [AUCblood]) was determined in the presence and absence of cyclosporine., Results: The AUCbrain/AUCblood ratio of 11C-radioactivity was increased by 88% +/- 20% (1.02 +/- 0.18 versus 0.55 +/- 0.10, P < .001) in the presence of cyclosporine (mean blood concentration, 2.8 +/- 0.4 micromol/L) without affecting 11C-verapamil metabolism or plasma protein binding. The corresponding increases for the brain white and gray matter were 84% +/- 13% and 84% +/- 18%, respectively., Conclusions: This is the first time that P-gp activity at the human BBB has been measured. The modest inhibition of human BBB P-gp by cyclosporine has implications for P-gp-based drug interactions at the human BBB. Our method for imaging P-gp activity can be used to identify multidrug-resistant tumors or to determine the contribution of P-gp polymorphism, inhibition, or induction to interindividual variability in drug response.

We sought to identify magnetic resonance imaging (MRI) parameters that can identify as well as predict disruption of the blood-brain barrier (BBB) after embolic stroke in the rat. Rats subjected to embolic stroke with (n=13) and without (n=13) rt-PA treatment were followed with MRI using quantitative permeability-related parameters, consisting of: transfer constant (K(i)) of Gd- DTPA, the distribution volume (V(p)) of the mobile protons, and the inverse of the apparent forward transfer rate for magnetization transfer (k(inv)), as well as the apparent diffusion coefficient of water (ADC(w)), T2, and cerebral cerebral blood flow (CBF). Tissue progressing to fibrin leakage resulting from BBB disruption and adjacent tissue were then analyzed to identify MRI markers that characterize BBB disruption. Animals were killed after final MRI measurements at 24 h after induction of embolic stroke and cerebral tissues were perfused and stained to detect fibrin leakage. K(i), V(p), and k(inv) were the most sensitive early (2 to 3 h) indices of the cerebral tissue that progresses to fibrin leakage. Cerebral blood flow was not significantly different between ischemic tissue with a compromised and an intact BBB. Our data indicate that compromise of the BBB can be sensitively predicted using a select set of MR parameters.

Unlabelled: Overexpression of multidrug resistance (MDR1) P-glycoprotein (Pgp) remains an important barrier to successful chemotherapy in cancer patients and impacts the pharmacokinetics of many important drugs, thus evoking a need to noninvasively interrogate Pgp transport activity in vivo., Methods: Cell tracer transport experiments as well as mouse biodistribution and microPET imaging studies were performed to characterize a nonmetabolized gallium(III) complex, gallium(III)-(bis(3-ethoxy-2-hydroxy-benzylidene)-N,N'-bis(2,2-dimethyl-3-amino-propyl)ethylenediamine) (Ga-[3-ethoxy-ENBDMPI])(+), as a candidate SPECT ((67)Ga) and generator-produced PET ((68)Ga) radiopharmaceutical recognized by MDR1 Pgp., Results: The (67)Ga-complex showed high membrane potential-dependent accumulation in drug-sensitive KB3-1 cells and modulator-reversible low accumulation in MDR KB8-5 cells. In KB8-5 cells, the median effective concentrations (EC(50)) of MDR modulators LY335979, PSC 833, and cyclosporin A were 69 nmol/L, 1 micromol/L, and 3 micromol/L, respectively. Using a variety of cells stably expressing MDR1 Pgp, multidrug resistance-associated proteins (MRP1-MRP6), or the breast cancer resistance protein (BCRP/MXR), the (67)Ga-complex was shown to be readily transported by MDR1 Pgp and, to a much lesser extent, by MRP1, but not MRP2-MRP6 or BCRP/MXR. In a nude mouse xenograft tumor model, the (67)Ga-complex produced a readily detected 3-fold difference between Pgp-expressing tumors and drug-sensitive tumors in the opposite flank. In mdr1a/1b(-/-) gene-deleted mice, the (67)Ga-complex showed 17-fold greater brain uptake and retention compared with wild-type mice with no net difference in blood pharmacokinetics, consistent with transport in vivo by Pgp expressed at the capillary blood-brain barrier. This could be readily observed with microPET using the (68)Ga-complex. Incidentally, wild-type mice showed heart-to-blood ratios of >100 by 1 h after injection and heart-to-liver ratios of 2.2 by 120 min., Conclusion: Molecular imaging of the functional transport activity of MDR1 Pgp with ((67/68)Ga-[3-ethoxy-ENBDMPI])(+) may enable noninvasive SPECT/PET monitoring of the blood-brain barrier, chemotherapeutic regimens, and MDR1 gene therapy protocols in vivo. These Pgp-directed properties of the radiopharmaceutical may also translate favorably to myocardial perfusion imaging.

The authors previously provided evidence of synchrotron radiation computed tomography (SRCT) efficacy for quantitative in vivo brain perfusion measurements using monochromatic X-ray beams. However, this technique was limited for small-animal studies by partial volume effects. In this paper, high-resolution absolute cerebral blood volume and blood-brain barrier permeability coefficient measurements were obtained on a rat glioma model using SRCT and a CCD camera (47x47 microm2 pixel size). This is the first report of in vivo high-resolution brain vasculature parameter assessment. The work gives interesting perspectives to quantify brain hemodynamic changes accurately in healthy and pathological small animals.

There is evidence that P-glycoprotein (P-gp) in the blood-brain barrier (BBB) may be involved in the aetiology of neurological disorders. For quantification of P-gp function in vivo, (R)-[11C]verapamil can be used as a positron emission tomography (PET) tracer, provided that a mathematical model describing kinetics of uptake and clearance of verapamil is available. To develop and validate such a model, the kinetic profile and metabolism of (R)-[11C]verapamil have to be known. The aim of this study was to investigate the presence of labeled metabolites of [11C]verapamil in the plasma and (brain) tissue of Wistar rats. For this purpose, extraction and high-performance liquid chromatography (HPLC) methods were developed. The radioactive metabolites of (R)-[11C]verapamil in the liver were N-dealkylated compounds, O-demethylated compounds and a polar fraction formed from N-demethylation products of (R)-[11C]verapamil. Apart from this [11C] polar fraction, other radioactive metabolites of [11C]verapamil were not detected in the brain tissue. Thirty minutes after injection, unmetabolized (R)-[11C]verapamil accounted for 47% of radioactivity in the plasma and 69% in the brain. Sixty minutes after injection, unmetabolized (R)-[11C] verapamil was 27% and 48% in the plasma and the brain, respectively.

Pigs of three different age groups (newborns, 1 week old, 6 weeks old) were used to study the transport of the large neutral amino acids 6-[18F]fluoro-L-DOPA ([18F]FDOPA) and 3-O-methyl-6-[18F]fluoro-L-DOPA ([18F]OMFD) across the blood-brain barrier (BBB) with positron emission tomography (PET). Compartmental modeling of PET data was used to calculate the blood-brain clearance (K1) and the rate constant for the brain-blood transfer (k2) of [18F]FDOPA and [18F]OMFD after i.v. injection. A 40-70% decrease of K1(OMFD), K1(FDOPA) and k2(OMFD) from newborns to juvenile pigs was found whereas k2(FDOPA) did not change. Generally, K1(OMFD) and k2(OMFD) are lower than K1(FDOPA) and k2(FDOPA) in all regions and age groups. The changes cannot be explained by differences in brain perfusion because the measured regional cerebral blood flow did not show major changes during the first 6 weeks after birth. In addition, alterations in plasma amino acids cannot account for the described transport changes. In newborn and juvenile pigs, HPLC measurements were performed. Despite significant changes of single amino acids (decrease: Met, Val, Leu; increase: Tyr), the sum of large neutral amino acids transported by LAT1 remained unchanged. Furthermore, treatment with a selective inhibitor of the LAT1 transporter (BCH) reduced the blood-brain transport of [18F]FDOPA and [18F]OMFD by 35% and 32%, respectively. Additional in-vitro studies using human LAT1 reveal a much lower affinity of FDOPA compared to OMFD or L-DOPA. The data indicate that the transport system(s) for neutral amino acids underlie(s) developmental changes after birth causing a decrease of the blood-brain barrier permeability for those amino acids during brain development. It is suggested that there is no tight coupling between brain amino acid supply and the demands of protein synthesis in the brain tissue.

Objective: The blood-brain barrier (BBB) is often perceived as a passive membrane. However, evidence has demonstrated that the BBB plays an active role in normal homeostasis and in certain disease processes., Methods: Approximately 300 peer-reviewed publications that discussed normal or abnormal BBB function were reviewed., Results: The role of the BBB and how it contributes to disorders of the central nervous system vary, depending on the specific disease process., Conclusion: In health and disease and extending to old age, endothelial cells, neurons, and glia constitute a neurovascular unit that regulates the BBB. Advances toward penetrating the BBB must account for both normal and abnormal functions of the neurovascular unit.

Permeability of the blood-brain barrier (BBB) is one of the factors determining the bioavailability of therapeutic drugs. The BBB only allows entry of lipophilic compounds with low molecular weights by passive diffusion. However, many lipophilic drugs show negligible brain uptake. They are substrates for transporters such as P-glycoprotein (P-gp), multidrug-resistance associated protein (MRP) and organic anion transporting polypeptides (OATPs). The action of these carrier systems results in rapid efflux of xenobiotics from the central nervous system (CNS). Classification of candidate drugs as substrates or inhibitors of such carrier proteins is of crucial importance in drug development. Positron emission tomography (PET) can play an important role in the screening process by providing in vivo information, after the putative drug has passed in vitro tests. Although radiolabeled probes for MRP and OATP function are not yet available, many radiotracers have been prepared to study P-glycoprotein function in vivo with PET. These include alkaloids ((11)C-colchicine), antineoplastic agents ((11)C-daunorubicin, (18)F-paclitaxel), modulators of L-type calcium channels ((11)C-(+/-)verapamil, (11)C-R(+)-verapamil), beta-adrenoceptor antagonists ((11)C-(S)-carazolol, (18)F-(S)-1'-fluorocarazolol, (11)C-carvedilol), serotonin 5-HT(1A) receptor antagonists ((18)F-MPPF), opioid receptor antagonists ((11)C-loperamide, (11)C-carfentanyl), and various (64)Cu-labeled copper complexes. Studies in experimental animals have indicated that it is possible to assess P-glycoprotein function in the BBB and its effect on the uptake and binding of drugs within the intact CNS, using suitable P-gp modulators labeled with positron emitters. Provided that radiopharmaceuticals (and P-gp modulators) can be developed for human use, several exciting fields of study may be explored, viz. (i) direct evaluation of the effect of modulators on the cerebral uptake of therapeutic drugs; (ii) assessment of mechanisms underlying drug resistance in epilepsy; (iii) examination of the role of the BBB in the pathophysiology of neurodegenerative and affective disorders; and (iv) exploration of the relationship between polymorphisms of transporter genes and the pharmacokinetics of test compounds within the CNS.

Unlabelled: We conducted a study on humans to determine whether quantitative evaluation of blood-brain barrier (BBB) breakdown using the (99m)Tc-diethylenetriaminepentaacetic acid ((99m)Tc-DTPA) SPECT technique at the peak time of stroke evolution can predict neurologic outcome., Methods: Thirty consecutive patients with acute stroke of the middle cerebral artery occurring >24 h and <48 h before admission were included in the study. Each patient underwent a complete neurologic examination according to the Scandinavian stroke score at 72 h after the stroke (S1) and again at 30 d (S2). The difference between initial and late scores was calculated (Delta S) and used to evaluate the change in neurologic status. A CT scan was obtained on all patients to determine the volume of stroke. The integrity of the BBB was evaluated using (99m)Tc-DTPA brain SPECT. A quantitative index of BBB disruption was defined as the ratio of the mean counts/pixel in the infarcted region compared with the mean counts/pixel in the contralateral nonaffected hemisphere. SPECT perfusion imaging was also performed with (99m)Tc-hexamethylpropyleneamine oxime ((99m)Tc-HMPAO) at 24 h after the (99m)Tc-DTPA study. The relative perfusion in the infarct region was expressed as the percentage of contralateral perfusion., Results: The mean (99m)Tc-DTPA disruption index was 6.8 +/- 6.9 (range, 1-26.2). Seven patients (23%) had no BBB disruption. Statistical analysis showed that the disruption index was negatively correlated with Delta S (r = -0.423, P < 0.02). A disruption index of <2.5 was associated with a significantly better neurologic outcome (mean Delta S, 17.5 +/- 9.5) compared with patients with an index of >2.5 (mean Delta S, -0.85 +/- 4.97, P < 0.0001) with a sensitivity of 95% and a specificity of 89%. S2 was significantly correlated with S1 (r = 0.738, P < 0.001) and with Delta S (r = 0.656, P < 0.001). Perfusion abnormalities on the (99m)Tc-HMPAO SPECT studies ranged between 12% and 90% (mean, 37.6% +/- 17.8%) compared with those on the contralateral nonaffected side. No correlation was found between (99m)Tc-HMPAO uptake and Delta S, infarct volume by CT, or disruption index. The CT volume measurements were negatively correlated with S2 (r = -0.560, P < 0.004) but not with Delta S., Conclusion: (99m)Tc-DTPA SPECT of the BBB combined with quantitative analysis in patients with acute stroke is significantly related to clinical outcome, with a distinct prognostic cutoff threshold of 2.5. The use of this radionuclide brain SPECT technique represents a unique application of conventional nondiffusible brain agents.

The mdr1 gene product P-glycoprotein (P-gp) is involved in the bioavailability and pharmacokinetics of various drugs. Racemic [(11)C]verapamil has been used to image P-gp expression in vivo. A racemic tracer, however, is not suitable for quantification. The purpose of the present study was to identify the most appropriate enantiomer of [(11)C]verapamil as a potential PET-tracer for quantifying P-gp function. The two enantiomers, (R)- and (S)-[(11)C]verapamil, were synthesized and studied in vivo. For the in vivo model mdr1a/1b double gene knock-out and wild type mice were used. The in vitro study made use of the LLC-PK1 MDR cell line to examine the P-gp mediated transport of both enantiomers. The biodistribution of (R)- and (S)-[(11)C]verapamil in dKO and WT mice demonstrated no stereoselectivity of verapamil for P-gp in the blood-brain barrier and in the testes. In addition, no significant differences in P-gp transport for both enantiomers were observed in the in vitro experiments. Previous studies have shown that (R)-verapamil is metabolized less in man and that it has lower affinity for calcium channels. Since (R)- and (S)-verapamil have equal transport for P-gp, the (R)-enantiomer seems to be the best and safest candidate as PET-tracer for measuring P-gp function in vivo.